Before that, the atom served as an increasingly useful hypothetical construct. At the same time, Einstein defined a new entity: a particle of light, the “light quantum,” now called the photon. Until then, everyone considered light to be a kind of wave. It didn’t bother Einstein that no one could observe this new thing. “It is the theory which decides what we can observe,” he said.
Adam Becker’s “What Is Real?” explores the controversy around quantum physics and its